Input switching in a ventricular intracardiac pacemaker

ABSTRACT

An intracardiac pacemaker system is configured to produce physiological atrial event signals by a sensing circuit of a ventricular intracardiac pacemaker and select a first atrial event input as the physiological atrial event signals. The ventricular intracardiac pacemaker detects atrial events from the selected first atrial event input, determines if input switching criteria are met, and switches from the first atrial event input to a second atrial event input in response to the input switching criteria being met. The second atrial event input includes broadcast atrial event signals produced by a second implantable medical device and received by the ventricular intracardiac pacemaker.

TECHNICAL FIELD

The disclosure relates to an intracardiac pacing system including aventricular pacemaker and an associated method for switching betweenatrial event inputs used for detecting atrial events and controllingatrial-synchronized ventricular pacing pulses by the ventricularpacemaker.

BACKGROUND

Implantable cardiac pacemakers are often placed in a subcutaneous pocketand coupled to one or more transvenous medical electrical leads carryingpacing and sensing electrodes positioned in the heart. A cardiacpacemaker implanted subcutaneously may be a single chamber pacemakercoupled to one transvenous medical lead for positioning electrodes inone heart chamber, atrial or ventricular, or a dual chamber pacemakercoupled to two intracardiac leads for positioning electrodes in both anatrial and a ventricular chamber. Multi-chamber pacemakers are alsoavailable that may be coupled to three leads, for example, forpositioning electrodes for pacing and sensing in one atrial chamber andboth the right and left ventricles.

Intracardiac pacemakers have recently been introduced that areimplantable within a ventricular chamber of a patient's heart fordelivering ventricular pacing pulses. Such a pacemaker may sense R-wavesignals attendant to intrinsic ventricular depolarizations and deliverventricular pacing pulses in the absence of sensed R-waves. While singlechamber ventricular sensing and pacing by an intracardiac ventricularpacemaker may adequately address some patient conditions, otherconditions may require atrial and ventricular (dual chamber) sensing forproviding atrial-synchronized ventricular pacing in order to maintain aregular heart rhythm.

SUMMARY

In general, the disclosure is directed to a intracardiac pacemakersystem and techniques for selecting an atrial event input used by aventricular intracardiac pacemaker for detecting atrial events andcontrolling atrial-synchronized ventricular pacing. A system operatingaccording to the techniques disclosed herein switches from a firstatrial event input to a second atrial event input in response to inputswitching criteria being met. The first atrial event input may bephysiological atrial event signals produced by a sensing circuit of theventricular intracardiac pacemaker. The second atrial event input may bebroadcast atrial event signals produced by a second implantable medicaldevice, which may be an atrial intracardiac pacemaker. Input switchingcriteria may include a loss of detected atrial events for one or moreventricular cycles, a threshold patient heart rate, a patient postureknown to be associated with unreliable atrial event detection, athreshold level of patient physical activity or any combination thereof.

In one example, the disclosure provides an intracardiac pacemaker systemincluding a ventricular intracardiac pacemaker having a pulse generator,a sensing circuit, a receiving circuit, and a control circuit coupled tothe pulse generator, the sensing circuit, and the receiving circuit. Thepulse generator is configured to generate and deliver pacing pulses to aventricle of a patient's heart via electrodes coupled to the ventricularintracardiac pacemaker. The sensing circuit is configured to producephysiological atrial event signals. The receiving circuit is configuredto receive broadcast atrial event signals that are broadcast by a secondimplantable medical device. The control circuit is configured to selecta first atrial event input as the physiological atrial event signals,detect first atrial events from the selected first atrial event input,determine if input switching criteria are met, switch from the firstatrial event input to a second atrial event input in response to theinput switching criteria being met, the second atrial event input beingthe broadcast atrial event signals. The control circuit is configured todetect second atrial events from the second atrial event input and setan atrioventricular (AV) pacing interval in response to detecting eachof the first atrial events and the second atrial events for controllingthe pulse generator to deliver the ventricular pacing pulses.

In another example, the disclosure provides a method performed by anintracardiac pacemaker system. The method includes producingphysiological atrial event signals by a sensing circuit of a ventricularintracardiac pacemaker and selecting a first atrial event input by acontrol circuit of the ventricular intracardiac pacemaker as thephysiological atrial event signals. The method further includesdetecting first atrial events from the selected first atrial eventinput, determining if input switching criteria are met and switchingfrom the first atrial event input to a second atrial event input inresponse to the input switching criteria being met. The second atrialevent input includes broadcast atrial event signals that are broadcastby a second implantable medical device and received by a receivingcircuit of the ventricular intracardiac pacemaker. The method furtherincludes detecting second atrial events from the second atrial eventinput and setting an AV pacing interval in response to detecting each ofthe first atrial events and the second atrial events for controlling apulse generator of the ventricular intracardiac pacemaker to deliverpacing pulses to a ventricle of a patient's heart via electrodes coupledto the ventricular intracardiac pacemaker.

In another example, the disclosure provides a non-transitory,computer-readable medium storing a set of instructions, which, whenexecuted by control circuitry of an implantable medical device systemincluding a ventricular intracardiac pacemaker and a second implantablemedical device, cause the system to produce physiological atrial eventsignals by a sensing circuit of the ventricular intracardiac pacemaker,select a first atrial event input received as the physiological atrialevent signals, detect first atrial events from the selected first atrialevent input, determine if input switching criteria are met and switchfrom the first atrial event input to a second atrial event input inresponse to the input switching criteria being met. The second atrialevent input includes broadcast atrial event signals that are broadcastby the second implantable medical device and received by the ventricularintracardiac pacemaker. The system is further caused to detect secondatrial events from the second atrial event input and set an AV pacinginterval in response to detecting each of the first atrial events andthe second atrial events for controlling a pulse generator of theventricular intracardiac pacemaker to deliver pacing pulses to aventricle of a patient's heart via electrodes coupled to the ventricularintracardiac pacemaker.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem that may be used to sense cardiac signals and deliveratrial-synchronized ventricular pacing pulses.

FIG. 2A is a conceptual diagram of an intracardiac pacemaker, which maycorrespond to the right atrial (RA) pacemaker or right ventricular (RV)pacemaker shown in FIG. 1.

FIG. 2B is a conceptual diagram of another example of intracardiacpacemaker.

FIG. 3 is a schematic diagram of an example configuration of the RVpacemaker shown in FIG. 1.

FIG. 4 is a schematic diagram of the RA pacemaker of FIG. 1 according toone example.

FIG. 5 is an example of a motion sensor signal that may be produced by amotion sensor of the RV pacemaker.

FIG. 6 is a flow chart of a method performed by the intracardiac pacingsystem of FIG. 1 for delivering atrial-synchronized ventricular pacingaccording to one example.

FIG. 7 is a timing diagram depicting cardiac events that are sensed bythe RA pacemaker and the RV pacemaker of FIG. 1 during ventricularpacing.

FIG. 8 is a timing diagram of atrial events and ventricular eventsproduced and detected by the intracardiac pacemaker system of FIG. 1.

FIG. 9 is a flow chart of a method performed by the RV pacemaker of FIG.1 for controlling atrial event input during atrial-synchronizedventricular pacing according to one example.

FIG. 10 is a flow chart of a method performed by the RA pacemaker ofFIG. 1 for controlling broadcasting of atrial event signals to the RVpacemaker.

DETAILED DESCRIPTION

During atrial-synchronized ventricular pacing, ventricular pacing pulsesare delivered at an AV pacing interval following an atrial event toprovide proper hemodynamic synchrony between the atrial contraction andthe ventricular contraction. In order for a ventricular intracardiacpacemaker to provide atrial-synchronized ventricular pacing, theventricular intracardiac pacemaker needs to sense or detect an atrialevent on each cardiac cycle to start the AV pacing interval. As theatrial rate increases or decreases during normal physiological heartrate changes, the ventricular pacing rate tracks the atrial rate. Thisatrial-synchronized ventricular pacing is referred to as an“atrial-tracking” pacing mode. In a non-atrial tracking ventricularpacing mode, or when an atrial event is not detected for starting the AVpacing interval, the ventricular pacing pulses are delivered at aventricular lower rate (LR) pacing interval that is independent ofatrial events and does not track the atrial rate but does preventventricular asystole in a patient with AV block. Generally,atrial-synchronized ventricular pacing mode is desirable over anon-atrial tracking pacing when the sinus node of the heart isfunctioning normally in setting the intrinsic atrial rate or the atriaare being paced at an appropriate pacing rate for the patient's level ofphysical activity.

In order to provide optimal synchrony between atrial systolic events andventricular systole, a ventricular intracardiac pacemaker needs toreceive reliable atrial event input signals indicative of the timing ofthe atrial systolic event. Atrial event input signals may be atrialP-wave signals included in the cardiac electrical signal received by theventricular intracardiac pacemaker, atrial mechanical event signalsincluded in a motion sensor signal such as an accelerometer signal,far-field atrial pacing pulse signals included in the cardiac electricalsignal received by the ventricular intracardiac pacemaker, or wirelesscommunication signals transmitted from an atrial intracardiac pacemakerto the ventricular intracardiac pacemaker. The detection of thesevarious atrial event input signals may have different power requirementsand differing reliability over time, between patients, and compared toeach other.

For example, receiving a wireless communication signal on a beat-by-beatbasis may require considerable more power than sensing a P-wave from thecardiac electrical signal received by the ventricular intracardiacpacemaker. P-wave sensing by a ventricular intracardiac pacemaker,however, may be challenging due to the relatively small far-field P-wavesignal amplitude compared to the near-field R-wave signal amplitude.Atrial systolic mechanical events detected from a motion sensor signalmay be reliable when the patient is at rest, but, during periods ofincreased patient physical activity or exercise, detection of atrialmechanical event signals may be confounded by patient physical activitysignals included in the motion sensor signal. Techniques are disclosedherein for selecting an atrial event input and switching between atrialevent inputs for providing efficient and reliable atrial event detectionand atrial tracking by ventricular pacing pulses delivered by aventricular intracardiac pacemaker.

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem 10 that may be used to sense cardiac signals and deliveratrial-synchronized ventricular pacing pulses. IMD system 10 includes aright ventricular (RV) intracardiac pacemaker 14 and a right atrial (RA)intracardiac pacemaker 12 in some examples. Pacemakers 12 and 14 aretranscatheter intracardiac pacemakers which may be adapted forimplantation wholly within a heart chamber, e.g., wholly within the RV,wholly within the left ventricle (LV), wholly within the RA or whollywithin the left atrium (LA) of heart 8. In the examples describedherein, the pacemaker system 10 is configured to sense cardiacelectrical signals and cardiac mechanical signals and provide pacingtherapy to a patient's heart 8. In particular, RV pacemaker 14 isconfigured to detect atrial events such as P-waves from a cardiacelectrical signal received by RV pacemaker 14 and/or detect atrialmechanical events from a motion signal produced by a motion sensorincluded in RV pacemaker 14.

Pacemakers 12 and 14 are reduced in size compared to subcutaneouslyimplanted pacemakers and may be generally cylindrical in shape to enabletransvenous implantation via a delivery catheter. In the example of FIG.1, RA pacemaker 12 is positioned along an endocardial wall of the RA,e.g., along the RA lateral wall or RA septum. RV Pacemaker 14 ispositioned along an endocardial wall of the RV, e.g., near the RV apexthough other locations are possible. The techniques disclosed herein arenot limited to the pacemaker locations shown in the example of FIG. 1and other positions and relative locations in the heart 8 and from eachother are possible. For example, RV pacemaker 14 may alternatively bepositioned in the LV and configured to detect cardiac signals anddeliver atrial-synchronized ventricular pacing to the LV using thetechniques disclosed herein. RA pacemaker 12 may be positioned outsideor within the right atrium or left atrium to provide respective rightatrial or left atrial sensing and pacing.

Pacemakers 12 and 14 are each capable of producing electricalstimulation pulses, e.g., pacing pulses, delivered to heart 8 via one ormore electrodes on the outer housing of the respective pacemaker. RApacemaker 12 is configured to deliver RA pacing pulses and sense acardiac electrical signal from within the RA that may be used to producean RA intracardiac electrogram (EGM) signal. RV pacemaker 14 isconfigured to deliver RV pacing pulses and sense an RV cardiacelectrical signal using housing based electrodes for producing an RV EGMsignal. The cardiac electrical signals may be sensed by the respectivepacemaker 12 or 14 using housing based electrodes that are also used todeliver pacing pulses to the respective heart chamber.

In some examples, a patient may only require RV pacemaker 14 fordelivering ventricular pacing. In other examples, depending onindividual patient need, RA pacemaker 12 may be required for deliveringatrial pacing. The RV pacemaker 14 is configured to control the deliveryof ventricular pacing pulses to the ventricle in a manner that promotessynchrony between RA activation and RV activation, e.g., by maintaininga target AV pacing interval between atrial events and ventricular pacingpulses. RV pacemaker 14 starts an AV pacing interval upon detecting anatrial event signal from an atrial event input corresponding to atrialsystole (intrinsic or paced) and delivers the ventricular pacing pulseupon expiration of the AV pacing interval to cause ventriculardepolarization.

A target AV pacing interval may be a programmed value selected by aclinician and is the time interval from the detection of the atrialevent until delivery of the ventricular pacing pulse. The target AVpacing interval may be identified as being hemodynamically optimal for agiven patient based on clinical testing or assessments of the patient orbased on clinical data from a population of patients. The target AVpacing interval may be determined to be optimal based on relative timingof electrical and/or mechanical events as identified from cardiacelectrical signals and/or motion sensor signals. As described below, thetarget AV pacing interval may be adjusted based on the atrial eventinput used by a control circuit of RV pacemaker 14 for detecting theatrial events since the time of atrial event detection may be dependenton the type of atrial event input signal being used.

According to the techniques described herein, atrial events may bedetected by RV pacemaker 14 from a motion sensor signal that includesmotion signals caused by ventricular and atrial events. For example,acceleration of blood flowing into the RV through the tricuspid valve 16between the right atrium and right ventricle caused by atrial systole,sometimes referred to as the “atrial kick,” may be detected by RVpacemaker 14 from the signal produced by a motion sensor, for example anaccelerometer, included in RV pacemaker 14.

Atrial events may be detected as far field atrial P-waves that areattendant to atrial depolarization may be detected by a control circuitof RV pacemaker 14 from the digitized cardiac electrical signal producedby a cardiac signal sensing circuit of RV pacemaker 14. Far-fieldP-waves are relatively low amplitude signals in the RV cardiacelectrical signal, e.g., compared to the near-field R-wave, andtherefore can be difficult to reliably detect at least some of the time.As such, atrial-synchronized ventricular pacing by RV pacemaker 14 mayrequire alternative atrial event input signals.

For example, in some cases, detection of far-field atrial pacing pulsesignals in the cardiac electrical signal produced by the cardiac signalsensing circuit of RV pacemaker 14 may provide more reliable atrialevent detection than P-wave sensing or detection of atrial mechanicalevents from a motion sensor signal. In some instances, RV pacemaker 14may be configured to switch from detecting P-waves in the cardiacelectrical signal to detecting atrial mechanical events in the motionsensor signal or to detecting far-field atrial pacing pulses in thecardiac electrical signal when P-wave sensing is determined to beunreliable based on input switching criteria.

RA pacemaker 12 and RV pacemaker 14 may be configured to communicatedirectly with each other via a wireless communication link 24. Whenpacemakers 12 and 14 are configured to communicate with each other,communication may be minimized in order to conserve battery life of theintracardiac pacemakers 12 and 14. As such, wireless telemetriccommunication may not occur on a beat-by-beat basis between the RApacemaker 12 and RV pacemaker 14 for communicating when the otherpacemaker is sensing cardiac events or when it is delivering pacingpulses. As disclosed herein, however, intracardiac pacing system 10 maybe configured to switch between atrial event inputs used by RV pacemaker14 for detecting the timing of atrial events and setting AV pacingintervals. As described below, if atrial event detection from a sensorsignal, e.g., from the cardiac electrical signal and/or from a motionsensor signal is lost or deemed unreliable, RV pacemaker 14 may switchthe atrial event input used for detecting atrial events to atrial eventsignals that are broadcast by RA pacemaker 12 via communication link 24.

In the examples described herein, RA pacemaker 12 is a secondimplantable medical device configured to broadcast atrial event signalsto RV pacemaker 14. In other examples, other implantable medical devicesmay be configured to broadcast atrial event signals to RV pacemaker 14,such as an implantable cardiac monitor capable of sensing P-waves, anICD, or other device capable of sensing atrial event signals andbroadcasting signals corresponding to the sensed atrial event signals.

Pacemakers 12 and 14 may each be capable of bidirectional wirelesscommunication with an external device 20 for programming sensing andpacing control parameters used by the respective pacemaker 12 or 14 forsensing atrial and ventricular events and for controlling the timing ofpacing pulse delivery. For example, pacemaker 14 may receive atrialevent sensing control parameters, the AV pacing intervals and otherpacing control parameters utilized for detecting the atrial events andfor delivering atrial-synchronized ventricular pacing. Other programmedparameters may relate to input switching criteria used by RV pacemaker14 for determining when to switch the atrial event input used fordetecting atrial events and atrial event broadcasting criteria used byRA pacemaker 12 for determining when to broadcast atrial event signalsto facilitate atrial event detection by RV pacemaker 14.

Different atrial event detection thresholds or parameters may beprogrammed for different atrial event inputs using programmer 20. Apaced AV pacing interval may be programmed for use after paced atrialevents, and a sensed AV pacing interval may be programmed for use aftersensed intrinsic atrial events. Furthermore, different AV pacingintervals may be programmed for use with different atrial event inputs.As such, up to four different paced AV (PAV) pacing intervals and up tofour different sensed AV (SAV) pacing intervals may be programmed whenthere are four different atrial event input signals available from whichatrial events may be detected.

Aspects of external device 20 may generally correspond to the externalprogramming/monitoring unit disclosed in U.S. Pat. No. 5,507,782(Kieval, et al.), hereby incorporated herein by reference in itsentirety. External device 20 is often referred to as a “programmer”because it is typically used by a physician, technician, nurse,clinician or other qualified user for programming operating parametersin pacemakers 12 and 14. External device 20 may be located in a clinic,hospital or other medical facility. External device 20 may alternativelybe embodied as a home monitor or a handheld device that may be used in amedical facility, in the patient's home, or another location.

External device 20 is configured for bidirectional communication withimplantable telemetry circuitry included in RV pacemaker 14 and RApacemaker 12. In some examples, external device 20 establishes awireless radio frequency (RF) communication link 22 with RA pacemaker 12and wireless RF communication link 26 with RV pacemaker 14 using acommunication protocol that appropriately addresses the targetedpacemaker 12 or 14. Communication links 22, 24 and 26 may be establishedusing an RF link such as BLUETOOTH®, Wi-Fi, Medical ImplantCommunication Service (MICS) or other communication bandwidth. Externaldevice 20 may include a programming head that is placed proximatepacemaker 12 or 14 to establish and maintain a communication link, andin other examples external device 20 and pacemakers 12 and 14 may beconfigured to communicate using a distance telemetry algorithm andcircuitry that does not require the use of a programming head and doesnot require user intervention to maintain a communication link. Anexample RF telemetry communication system that may be implemented insystem 10 is generally disclosed in U.S. Pat. No. 5,683,432 (Goedeke, etal.), hereby incorporated herein by reference in its entirety.

External device 20 may display data and information relating topacemaker functions to a user for reviewing pacemaker operation andprogrammed parameters as well as EGM signals transmitted from RVpacemaker 14 or RA pacemaker 12, motion sensor signals acquired by RApacemaker 14, or other physiological data that is acquired by andretrieved from pacemakers 12 and/or 14 during an interrogation session.

It is contemplated that external device 20 may be in wired or wirelessconnection to a communications network via a telemetry circuit thatincludes a transceiver and antenna or via a hardwired communication linefor transferring data to a remote database or computer to allow remotemanagement of the patient. Remote patient management systems including aremote patient database may be configured to utilize the presentlydisclosed techniques to enable a clinician to review EGM, motion sensor,and marker channel data and authorize programming of sensing and therapycontrol parameters in RA pacemaker 12 and/or RV pacemaker 14, e.g.,after viewing a visual representation of EGM, motion sensor signal andmarker channel data.

FIG. 2A is a conceptual diagram of an intracardiac pacemaker 100, whichmay correspond to RA pacemaker 12 or RV pacemaker 14 shown in FIG. 1.Pacemaker 100 includes electrodes 162 and 164 spaced apart along thehousing 150 of pacemaker 100 for sensing cardiac electrical signals anddelivering pacing pulses. Electrode 164 is shown as a tip electrodeextending from a distal end 102 of pacemaker 100, and electrode 162 isshown as a ring electrode along a mid-portion of housing 150, forexample adjacent proximal end 104. Distal end 102 is referred to as“distal” in that it is expected to be the leading end as pacemaker 14 isadvanced through a delivery tool, such as a catheter, and placed againsta targeted pacing site.

Electrodes 162 and 164 form an anode and cathode pair for bipolarcardiac pacing and sensing. In alternative embodiments, pacemaker 100may include two or more ring electrodes, two tip electrodes, and/orother types of electrodes exposed along pacemaker housing 150 fordelivering electrical stimulation to heart 8 and sensing cardiacelectrical signals. Electrodes 162 and 164 may be, without limitation,titanium, platinum, iridium or alloys thereof and may include a lowpolarizing coating, such as titanium nitride, iridium oxide, rutheniumoxide, platinum black among others. Electrodes 162 and 164 may bepositioned at locations along pacemaker 14 other than the locationsshown.

Housing 150 is formed from a biocompatible material, such as a stainlesssteel or titanium alloy. In some examples, the housing 150 may includean insulating coating. Examples of insulating coatings include parylene,urethane, PEEK, or polyimide among others. The entirety of the housing150 may be insulated, but only electrodes 162 and 164 uninsulated.Electrode 164 may serve as a cathode electrode and be coupled tointernal circuitry, e.g., a pacing pulse generator and cardiac signalsensing circuitry, enclosed by housing 150 via an electrical feedthroughcrossing housing 150. Electrode 162 may be formed as a conductiveportion of housing 150 as a ring electrode that is electrically isolatedfrom the other portions of the housing 150 as generally shown in FIG.2A. In other examples, the entire periphery of the housing 150 mayfunction as an electrode that is electrically isolated from tipelectrode 164, instead of providing a localized ring electrode such asanode electrode 162. Electrode 162 formed along an electricallyconductive portion of housing 150 serves as a return anode during pacingand sensing.

The housing 150 includes a control electronics subassembly 152, whichhouses the electronics for sensing cardiac signals, producing pacingpulses and controlling therapy delivery and other functions of pacemaker100 as described below in conjunction with FIG. 3. A motion sensor maybe implemented as an accelerometer enclosed within housing 150 in someexamples. The accelerometer provides a signal to a processor included incontrol electronics subassembly 152 for signal processing and analysisfor detecting cardiac mechanical events, e.g., atrial systolic events inRV pacemaker 14, and may be used for determining patient physicalactivity for providing rate responsive pacing.

Housing 150 further includes a battery subassembly 160, which providespower to the control electronics subassembly 152. Battery subassembly160 may include features of the batteries disclosed in commonly-assignedU.S. Pat. No. 8,433,409 (Johnson, et al.) and U.S. Pat. No. 8,541,131(Lund, et al.), both of which are hereby incorporated by referenceherein in their entirety.

Pacemaker 100 may include a set of fixation tines 166 to securepacemaker 100 to patient tissue, e.g., by actively engaging with theendocardium or interacting with the ventricular trabeculae. Fixationtines 166 are configured to anchor pacemaker 100 to position electrode164 in operative proximity to a targeted tissue for deliveringtherapeutic electrical stimulation pulses. Numerous types of activeand/or passive fixation members may be employed for anchoring orstabilizing pacemaker 100 in an implant position. Pacemaker 100 mayinclude a set of fixation tines as disclosed in pending U.S. PublicationNo. 2012/0172892 (Grubac, et al.), hereby incorporated herein byreference in its entirety.

Pacemaker 100 may optionally include a delivery tool interface 158.Delivery tool interface 158 may be located at the proximal end 104 ofpacemaker 100 and is configured to connect to a delivery device, such asa catheter, used to position pacemaker 100 at an implant location duringan implantation procedure, for example within a heart chamber.

FIG. 2B is a conceptual diagram of another example of intracardiacpacemaker 100. In FIG. 2B, pacemaker 100 includes a proximal sensingextension 165 extending away from housing 150 and carrying one or more,in this case a pair, of sensing electrodes 167 and 168. The proximalsensing extension 165 may be coupled to the housing 150 for positioninga return sensing electrode 168 or 167 which may be paired with distalelectrode 164 at an increased inter-electrode distance compared to theinter-electrode spacing of housing-based electrodes 162 and 164. Theincreased inter-electrode distance may facilitate sensing of far-fieldcardiac signals. For example, pacemaker 100 having sensing extension 165may correspond to RV pacemaker 14 of FIG. 1. When distal end 102 isfixed along the RV apex, sensing extension 165 may extend toward the RAthereby positioning electrodes 167 and 168 nearer the atrial tissue forsensing far-field atrial P-waves. Electrode 164 may be used with anelectrode 167 carried by sensing extension 165 for obtaining a cardiacelectrical signal including far-field P-waves when pacemaker 100 ispositioned in the RV. Alternatively, electrodes 167 and 168 may form asensing electrode pair. One electrode 167 may be coupled to sensingcircuitry enclosed in housing 150 via an electrical feedthrough crossinghousing 150, and one electrode 168 may be coupled to housing 150 toserve as a ground electrode.

FIG. 3 is a schematic diagram of an example configuration of RVpacemaker 14 shown in FIG. 1. In this example, RV pacemaker 14 includesa pulse generator 202, a cardiac signal sensing circuit 204, a controlcircuit 206, telemetry circuit 208, memory 210 and a power source 214.The various circuits represented in FIG. 3 may be combined on one ormore integrated circuit boards which include a specific integratedcircuit (ASIC), an electronic circuit, a processor (shared, dedicated,or group) and memory that execute one or more software or firmwareprograms, a combinational logic circuit, state machine or other suitablecomponents that provide the described functionality.

Depiction of different features of RV pacemaker 14 as specific circuitryis intended to highlight different functional aspects and does notnecessarily imply that such functions must be realized by separatehardware, firmware or software components or by any particular circuitarchitecture. Rather, functionality associated with one or more circuitsdescribed herein may be performed by separate hardware, firmware orsoftware components, or integrated within common hardware, firmware orsoftware components. For example, atrial event detection from an atrialevent input and ventricular pacing control operations performed by RVpacemaker 14 may be implemented in control circuit 206 executinginstructions stored in memory 210 and relying on input from cardiacsignal sensing circuit 204 and/or telemetry circuit 208. Providingsoftware, hardware, and/or firmware to accomplish the describedfunctionality in the context of any modern pacemaker, given thedisclosure herein, is within the abilities of one of skill in the art.

Control circuit 206 includes an atrial event detector circuit 240, pacetiming circuit 242, and processor 244 for performing the variousfunctions attributed to control circuit 206 described herein. Controlcircuit 206 is configured to control pulse generator 202 to deliveratrial-synchronized ventricular pacing by setting an AV pacing intervalin response to atrial event detector circuit 240 detecting an atrialevent indicative of atrial systole. The atrial event is detected from aselected atrial event input received by atrial event detector circuit240 from cardiac signal sensing circuit 204 or telemetry circuit 208.Control circuit 206 may selectively control which one of multiple atrialevent inputs is used at any given time for detecting the atrial eventsto cause the AV pacing interval to be started.

Various atrial event input available for detection of atrial events byatrial event detector circuit 240 may include far-field P-waves includedin a cardiac electrical signal received by sensing circuit 204 viaelectrodes 162V and 164V (and/or electrodes 167 and 168 when a sensingextension 165 is included as shown in FIG. 2B), atrial mechanical eventsignals included in a motion signal produced by motion sensor 212 ofsensing circuit 204, far-field atrial pacing pulses included in acardiac electrical signal received by sensing circuit 204, and broadcastatrial event signals received by telemetry circuit 208. Atrial eventdetector circuit 240 detects an atrial event from the selected atrialevent input and passes an atrial event detection signal to pace timingcircuit 242. Pace timing circuit 242 starts the AV pacing interval inresponse to the atrial event detection signal and controls pulsegenerator 202 to deliver a ventricular pacing pulse via electrodes 162Vand 164V upon expiration of the AV pacing interval.

In this example, RV pacemaker 14 includes motion sensor 212 forproducing a motion signal that includes atrial systolic mechanical eventsignals which can be detected by atrial event detector circuit 240.Motion sensor 212 may be implemented as an accelerometer; however, othermotion sensors may be utilized successfully in pacemaker 14 fordetecting atrial systolic mechanical events. Examples of motion sensorsthat may be implemented in pacemaker 14 include piezoelectric sensorsand micro electro-mechanical systems (MEMS) devices. One example of anaccelerometer for use in implantable medical devices is generallydisclosed in U.S. Pat. No. 5,885,471 (Ruben, et al.), incorporatedherein by reference in its entirety. An implantable medical devicearrangement including a piezoelectric accelerometer for detectingpatient motion is disclosed in U.S. Pat. No. 4,485,813 (Anderson, etal.) and in U.S. Pat. No. 5,052,388 (Sivula, et al.), both of whichpatents are hereby incorporated by reference herein in their entirety.

Motion sensor 212 produces a motion signal that is correlated to motionor vibration of sensor 212 (and RV pacemaker 14), e.g., when subjectedto flowing blood and cardiac motion as well as patient physicalactivity. Motion sensor 212 may be a one-dimensional, single axisaccelerometer, two-dimensional or three-dimensional multi-axisaccelerometer. Each axis signal may be analyzed individually or incombination for detecting atrial systolic events. Examples ofthree-dimensional accelerometers that may be implemented in RV pacemaker14 and used for determining patient physical activity and detectingatrial mechanical systolic events for controlling ventricular pacingpulses using the presently disclosed techniques are generally describedin U.S. Pat. No. 5,593,431 (Sheldon) and U.S. Pat. No. 6,044,297(Sheldon), both of which are incorporated herein by reference in theirentirety.

Cardiac signal sensing circuit 204 (also referred to herein simply as“sensing circuit” 204) is configured to receive a cardiac electricalsignal via electrodes 162V and 164V by a pre-filter and amplifiercircuit 220. Pre-filter and amplifier circuit may include a high passfilter to remove DC offset, e.g., a 2.5 to 5 Hz high pass filter, or awideband filter having a passband of 2.5 Hz to 100 Hz to remove DCoffset and high frequency noise. Pre-filter and amplifier circuit 220may further include an amplifier to amplify the “raw” cardiac electricalsignal passed to analog-to-digital converter (ADC) 226. ADC 226 may passa multi-bit, digital electrogram (EGM) signal to control circuit 206 foruse by atrial event detector circuit 240 in detecting far-field atrialP-waves or detecting far-field atrial pacing pulses. Sensing offar-field P-waves from the cardiac electrical signal received by controlcircuit 206 may be performed using methods generally disclosed in U.S.Pat. Publication No. 2016/0114169 A1 (Sheldon, et al.), published onApr. 28, 2016, incorporated herein by reference in its entirety. Thetechniques disclosed herein, however, are not limited to a particularmethod or circuitry for detecting P-wave signals produced by cardiacsignal sensing circuit 204 from the cardiac electrical signal receivedvia electrodes 162V and 164V coupled to RV pacemaker 14 and passed tocontrol circuit 206.

Sensing circuit 204 includes R-wave detector 224 for detecting R-wavesfrom the cardiac electrical signal. The digital signal from ADC 226 maybe passed to rectifier and amplifier circuit 222, which may include arectifier, bandpass filter, and amplifier for passing a cardiac signalto R-wave detector 224. R-wave detector 224 may include a senseamplifier or other detection circuitry that compares the incomingrectified, cardiac electrical signal to an R-wave detection threshold,which may be an auto-adjusting threshold. When the incoming signalcrosses the R-wave detection threshold, the R-wave detector 224 producesan R-wave sensed event signal (R-sense) that is passed to controlcircuit 206. In other examples, R-wave detector 224 may receive thedigital output of ADC 226 for detecting R-waves by a comparator,morphological signal analysis of the digital EGM signal or other R-wavedetection techniques. R-wave sensed event signals passed from R-wavedetector 224 to control circuit 206 may be used for inhibiting ascheduled pacing pulse and for starting a ventricular lower rate (LR)pacing interval for maintaining a minimum ventricular pacing rate in theabsence of detected atrial events.

The far-field P-waves or far-field atrial pacing pulses included thecardiac electrical signal received from sensing circuit 204 may beselected as the atrial event input used by atrial event detector circuit240 for detecting atrial events. Control circuit 206 may select theatrial event input as a physiological atrial event input to be used byatrial event detector circuit 240 for detecting atrial events. Forinstance, control circuit 206 may select a physiological atrial eventinput by enabling atrial event detector circuit 240 to detect far-fieldatrial P-waves from the cardiac electrical signal received from ADC 226.In other instances, control circuit 206 may select a physiologicalatrial event input by enabling atrial event detector circuit 240 todetect atrial systolic mechanical events from the motion signal producedby motion sensor 212. These physiological atrial event inputs, P-wavesignals or atrial systolic mechanical event signals, arise from aphysiological source (e.g., atrial myocardial depolarization or atrialmyocardial contraction) and do not require the RA pacemaker 12 togenerate or broadcast an atrial event signal that is detected directlyby atrial event detector circuit 240.

If atrial event detection from a physiological atrial event input islost, or if other input switching criteria are met, control circuit 206may control atrial event detector circuit 240 to use a different atrialevent input for detecting atrial events. The atrial event input may beswitched from a physiological atrial event input to a broadcast atrialevent input. Broadcast atrial event input is signals produced by RApacemaker 12 for direct detection by RV pacemaker 14. RV pacemaker 14includes a receiver circuit for receiving broadcast atrial event inputsignals. In the example of FIG. 3, telemetry circuit 208 is one receivercircuit that is configured to receive broadcast atrial event signals inthe form of wireless telemetry communication signals transmitted by RApacemaker 12.

Cardiac signal sensing circuit 204 may also act as a receiver circuitfor receiving broadcast atrial event signals in the form of far-fieldatrial pacing pulses. RA pacemaker 12 may broadcast atrial event signalsby delivering atrial pacing pulses for pacing the atria and/or duringthe physiological refractory period following a P-wave. The atrialpacing pulses may be delivered at an increased atrial pacing pulseenergy to increase the far-field atrial pacing pulse signal strength inthe cardiac electrical signal received by electrodes 162V and 164V andpassed to atrial event detector circuit 240.

As such, the term “receiver circuit” as used herein, refers to anycircuit of RV pacemaker 14 that is configured to pass an atrial eventinput to control circuit 206 that includes atrial event signals that arereceived from and broadcast by RA pacemaker 12. The broadcast atrialevent input signals are device-generated signals that are detecteddirectly by RV pacemaker 14 as opposed to the physiological atrial eventinput signals which arise from the atria as physiological electrical ormechanical signals. Physiological atrial event input signals may bepaced or intrinsic atrial events. In the case of paced atrial events,however, the physiological atrial event input signal is the evokedP-wave or the evoked atrial mechanical response to the atrial pacingpulse. The atrial pacing pulse is not detected directly by RV pacemaker14 when the physiological atrial event input is selected. In contrast,when atrial event detector 240 is enabled to detect atrial events from abroadcast atrial event input of far-field atrial pacing pulses, theatrial pacing pulses are detected directly from the cardiac electricalsignal received by sensing circuit 204 rather than detecting theelectrical or mechanical evoked response of the atria to the deliveredatrial pacing pulse. Detecting a paced atrial event from a physiologicalatrial event input may be thought of as an indirect detection of anatrial pacing pulse since the pacing-evoked P-wave or evoked mechanicalevent is detected but not the pacing pulse itself. The RV pacemaker 14,however, may be unable to distinguish between paced and sensed atrialevents when using a physiological atrial event input.

Control circuit 206 may receive R-wave sensed event signals, digitalcardiac electrical signals, and/or motion sensor signals from sensingcircuit 204 for use in detecting and confirming cardiac events andcontrolling ventricular pacing. For example, R-wave sensed event signalsmay be passed to pace timing circuit 242 for inhibiting a scheduledventricular pacing pulses and scheduling ventricular pacing pulses at aprogrammed ventricular LR pacing interval for preventing ventricularasystole in the absence of detected atrial events. The R-wave sensedevent signals may also be received by atrial event detector circuit 240for setting various blanking periods, refractory periods or sensingwindows that are applied to an atrial event input for facilitatingatrial event detection from the selected atrial event input.

Processor 244 may include one or more clocks for generating clocksignals that are used by atrial event detector circuit 240 and by pacetiming circuit 242 to control the timing of sensing windows, refractoryperiods, and for timing out a pacing escape interval that may be set tothe AV pacing interval that is started upon atrial event detection. Pacetiming circuit 242 may include one or more pacing escape interval timersor counters that are used to time out the AV pacing interval, which maybe a programmable interval stored in memory 210 and retrieved byprocessor 244 for use in setting the AV pacing interval used by pacetiming circuit 242. As described below, the AV pacing interval may beset based on the atrial event input selected for atrial event detection.

Pace timing circuit 242 may additionally include an escape intervaltimer for timing out a ventricular LR pacing interval timer forcontrolling a minimum ventricular pacing rate. The LR pacing interval isstarted upon an R-wave sensed event signal or a delivered ventricularpacing pulse. If an atrial systolic event is not detected by atrialevent detector circuit 240 during the LR pacing interval, a ventricularpacing pulse may be delivered by pulse generator 202 upon expiration ofthe LR pacing interval. The ventricular pacing pulse in this case is anon-atrial tracking or non-synchronous pacing pulse.

Processor 244 may retrieve other programmable pacing control parameters,such as pacing pulse amplitude and pacing pulse width that are used tocontrol pulse generator 202 in generating pacing pulses. In addition toproviding control signals to pace timing circuit 242 and pulse generator202 for controlling pacing pulse delivery, processor 244 may providesensing control signals to sensing circuit 204, e.g., R-wave sensingthreshold control parameters, R-wave sensitivity, various blanking andrefractory intervals applied to the cardiac electrical signal, andatrial event detection control signals to atrial event detector circuit240 for use in detecting and confirming atrial events, e.g., in settingevent detection windows, atrial refractory period, detection thresholdamplitudes applied to the atrial event input signals, and any otheratrial event detection criteria or parameters applied by circuitryincluded in atrial event detector circuit 240.

Pulse generator 202 generates electrical pacing pulses that aredelivered to the right ventricle of the patient's heart via cathodeelectrode 164V and return anode electrode 162V. Pulse generator 202 mayinclude charging circuit 230, switching circuit 232 and an outputcircuit 234. Charging circuit 230 may include a holding capacitor thatmay be charged to a pacing pulse amplitude by a multiple of the batteryvoltage signal of power source 214 under the control of a voltageregulator. The pacing pulse amplitude may be set based on a controlsignal from control circuit 206. Switching circuit 232 may control whenthe holding capacitor of charging circuit 230 is coupled to the outputcircuit 234 for delivering the pacing pulse. For example, switchingcircuit 232 may include a switch that is activated by a timing signalreceived from pace timing circuit 242 upon expiration of an AV pacinginterval (or a LR pacing interval) and kept closed for a programmedpacing pulse duration to enable discharging of the holding capacitor ofcharging circuit 230. The holding capacitor, previously charged to thepacing pulse voltage amplitude, is discharged across electrodes 164V and162V through the output capacitor of output circuit 234 for theprogrammed pacing pulse duration. Examples of pacing circuitry generallydisclosed in U.S. Pat. No. 5,507,782 (Kieval, et al.) and in commonlyassigned U.S. Pat. No. 8,532,785 (Crutchfield, et al.), both of whichpatents are incorporated herein by reference in their entirety, may beimplemented in RV pacemaker 14 for charging a pacing capacitor to apredetermined pacing pulse amplitude under the control of controlcircuit 206 and delivering a pacing pulse.

Memory 210 may include computer-readable instructions that, whenexecuted by control circuit 206, cause control circuit 206 to performvarious functions attributed throughout this disclosure to RV pacemaker14. The computer-readable instructions may be encoded within memory 210.Memory 210 may include any non-transitory, computer-readable storagemedia including any volatile, non-volatile, magnetic, optical, orelectrical media, such as a random access memory (RAM), read-only memory(ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM(EEPROM), flash memory, or other digital media with the sole exceptionbeing a transitory propagating signal.

Power source 214 provides power to each of the other circuits andcomponents of RV pacemaker 14 as required. Power source 214 may includeone or more energy storage devices, such as one or more rechargeable ornon-rechargeable batteries. The connections between power source 214 andother pacemaker circuits and components are not shown in FIG. 3 for thesake of clarity.

Telemetry circuit 208 includes a transceiver 209 and antenna 211 fortransferring and receiving data via a radio frequency (RF) communicationlink. Telemetry circuit 208 may be capable of bi-directionalcommunication with external device 20 (FIG. 1) as described above.Motion sensor signals and cardiac electrical signals, and/or dataderived therefrom may be transmitted by telemetry circuit 208 toexternal device 20. Programmable control parameters and algorithms forperforming atrial event detection and ventricular pacing control may bereceived by telemetry circuit 208 and stored in memory 210 for access bycontrol circuit 206.

Telemetry circuit 208 may be configured to at least receive wirelesscommunication signals from RA pacemaker 12 and may be configured forbi-directional communication with RA pacemaker 12. In some instances,broadcast atrial event signals may be transmitted from RA pacemaker 12to RV pacemaker telemetry circuit 208. Telemetry circuit 208 passes theatrial event signals to control circuit 206 for detection by atrialevent detector circuit 240. Control circuit 206 may control telemetrycircuit 208 to wake up to receive broadcast atrial event signals whencontrol circuit 206 determines that input switching criteria are met andselects broadcast atrial event input for atrial event detector circuit240. Telemetry circuit 208 may be powered up for receiving broadcastatrial event signals from RA pacemaker 12 until another atrial eventinput is selected for detecting atrial events.

The illustrative embodiments described herein include RA pacemaker 12for producing broadcast atrial event signals. It is contemplated,however, that other implantable medical devices may broadcast atrialevent signals. For example, a sensing-only device may be used to monitorfor atrial P-waves and transmit atrial event signals to RV pacemaker 14.Another implantable medical device capable of broadcasting atrial eventsignals may be an intracardiac or extracardiac implantable device.

FIG. 4 is a schematic diagram of RA pacemaker 12 according to oneexample. RA pacemaker 12 includes pulse generator 302, sensing circuit304, control circuit 306, telemetry circuit 308, memory 310, patientactivity sensor 312, and power source 314. As described above inconjunction with FIG. 3, the various circuits represented in FIG. 4 maybe combined on one or more integrated circuit boards which include aspecific integrated circuit (ASIC), an electronic circuit, a processor(shared, dedicated, or group) and memory that execute one or moresoftware or firmware programs, a combinational logic circuit, statemachine or other suitable components that provide the describedfunctionality. Providing software, hardware, and/or firmware toaccomplish the described functionality of RA pacemaker 12 in the contextof any modern pacemaker, given the disclosure herein, is within theabilities of one of skill in the art.

Control circuit 306 includes a ventricular event detector circuit 340,pace timing circuit 342, and processor 344 for performing the variousfunctions attributed to control circuit 306 described herein. Controlcircuit 306 is configured to control pulse generator 302 to deliveratrial pacing pulses according to an atrial LR pacing interval in theabsence of sensed, intrinsic atrial P-waves.

Sensing circuit 304 is configured to receive a cardiac electrical signalvia electrodes 162A and 164A for sensing atrial P-waves and far-fieldventricular R-waves. Sensing circuit 304 may include a pre-filter andamplifier circuit 320, ADC 326 and rectifier/amplifier 322 as generallydescribed above in conjunction with FIG. 3. Components of sensingcircuit 304 may be tuned to different filtering frequencies foroptimizing P-wave and far-field R-wave sensing than sensing circuit 204,which is configured for providing a signal for near-field R-wave sensingand far-field P-wave sensing.

Sensing circuit 304 includes P-wave detector 324 for detecting P-wavesfrom the rectified, filtered cardiac electrical signal by a senseamplifier or other detection circuitry that compares the incoming signalto a P-wave detection threshold, which may be an auto-adjustingthreshold. When the incoming signal crosses the P-wave detectionthreshold, the P-wave detector 324 produces a P-wave sensed event signal(P-sense) that is passed to control circuit 306. P-wave sensed eventsignals passed to control circuit 306 may be used for inhibiting ascheduled atrial pacing pulse and re-starting an atrial LR pacinginterval by pace timing circuit 342.

Control circuit 306 may receive P-wave sensed event signals and/ordigital cardiac electrical signals from sensing circuit 204 for use indetecting and confirming cardiac events and controlling atrial pacing.Processor 344 may receive a patient activity sensor signal from sensor312 for determining a patient activity metric and sensor indicatedpacing rate (SIR). The LR pacing interval set by pace timing circuit 342may be set according to the SIR to provide rate responsive pacing to theRA based on the patient's physical activity level.

Patient activity sensor 312 may be implemented as an accelerometer,which may be a piezoelectric accelerometer or a MEMS device. Processor344 may adjust the LR pacing interval set by pace timing circuit 342from a permanent LR pacing interval corresponding to a minimum atrialpacing rate to a temporary LR pacing interval to provide rate responsivepacing. The temporary LR pacing interval is set based on the SIRdetermined from the patient activity sensor signal to provide atrialpacing pulses at a rate greater than the minimum or base atrial pacingrate. The higher atrial rate support is provided according to thepatient's metabolic demand during periods of non-resting physicalactivity based on the SIR. The use of an accelerometer in anintracardiac pacemaker for obtaining a patient activity signal isgenerally disclosed in pre-grant U.S. Pat. Publication No. 2015/0217119A1 filed on Feb. 6, 2014 (Nikolski, et al.), incorporated herein byreference in its entirety. Examples of techniques for using a patientactivity signal for determining a SIR and providing rate-responsivepacing are generally disclosed in U.S. Pat. No. 5,720,769 (van Oort) andU.S. Pat. No. 7,031,772 (Condie, et al.), both incorporated herein byreference in its entirety. Example techniques that may be implemented inan intracardiac pacemaker for providing rate responsive pacing based onpatient activity are generally disclosed in pending U.S. Publication No.2015/0217119, (Sheldon, et al.) and U.S. patent application Ser. No.14/920,228 filed Oct. 22, 2015 (Sheldon, et al.).

RA pacemaker 14 includes a ventricular event detector circuit 340 shownincluded control 306 in the example of FIG. 4 for detecting ventricularevents, e.g., ventricular pacing pulses, evoked R-waves, and/orintrinsic R-waves. ADC 326 may pass a multi-bit, digital cardiacelectrical signal to control circuit 306 for use by ventricular eventdetector circuit 340 in detecting far-field ventricular R-waves and/orfar-field ventricular pacing pulses in some examples. Ventricular eventdetector circuit 340 may pass a ventricular event detection signal toprocessor 344 for determining a detected AV interval between a sensedP-wave or atrial pacing pulse and the subsequently detected ventricularevent. Control circuit 306 monitors detected AV intervals for verifyingthat RV pacemaker 14 is delivering ventricular pacing pulses at anexpected AV pacing interval as described in the timing diagrams and flowcharts presented herein.

If ventricular events are not detected at the expected AV pacinginterval, control circuit 306 may begin broadcasting atrial eventsignals. Control circuit 306 may control telemetry circuit 308 to begintransmitting atrial event signals to RV pacemaker 14 as wireless RFcommunication signals. Additionally or alternatively, control circuit306 may broadcast atrial event signals by controlling pulse generator302 to deliver atrial pacing pulses with a higher amplitude and/or pulsewidth to promote detection of atrial pacing pulses by atrial eventdetector 240 of RV pacemaker 14. In some cases, control circuit 306 maycontrol pulse generator 302 to deliver a refractory pacing pulse, duringthe physiological refractory period of the atria following a P-wave as abroadcast atrial event signal. RV pacemaker 14 may switch from using aphysiological atrial event input, such as P-wave signals or atrialmechanical event signals, to a broadcast atrial event input, e.g.,wireless communication signals transmitted by telemetry circuit 308 oratrial pacing pulses delivered by pulse generator 302 and detected asfar-field atrial pacing pulses by RV pacemaker 12. RA pacemaker 12 maybe configured to recognize when RV pacemaker 14 has lost atrial eventdetection or switched to a broadcast atrial event input and operate topromote continuity of atrial event detection by RV pacemaker 14 bybroadcasting atrial event signals.

Processor 344 may include one or more clocks for generating clocksignals that are used by pace timing circuit 342 to control the timingof the LR atrial pacing interval. Pace timing circuit 342 may includeone or more pacing escape interval timers or counters that are used totime out the LR pacing interval. Processor 344 may retrieve otherprogrammable pacing control parameters, such as pacing pulse amplitudeand pacing pulse width that are used to control pulse generator 302 ingenerating pacing pulses. In addition to providing control signals topace timing circuit 342 and pulse generator 302 for controlling pacingpulse delivery, processor 344 may provide sensing control signals tosensing circuit 304, e.g., P-wave sensing threshold parameters,sensitivity, various blanking and refractory intervals applied to thecardiac electrical signal, and ventricular event detection controlsignals to ventricular event detector circuit 340 for use in detectingventricular events and expected AV intervals for verifying atrial eventdetection by RV pacemaker 14 based on the expected timing of ventricularevents following a most recent atrial event, paced or sensed.

Pulse generator 302 generates electrical pacing pulses that aredelivered to the RA of the patient's heart via cathode electrode 164Aand return anode electrode 162A. Pulse generator 302 may include acharging circuit, switching circuit and an output circuit as generallydescribed above in conjunction with pulse generator 202 of FIG. 3 forgenerating and delivering atrial pacing pulses at timed LR intervalsunder the control of pace timing circuit 342.

Memory 310 may include computer-readable instructions that, whenexecuted by control circuit 306, cause control circuit 306 to performvarious functions attributed throughout this disclosure to RA pacemaker14. The computer-readable instructions may be encoded within anynon-transitory, computer-readable storage media listed above. RApacemaker control circuit 306 and RV pacemaker control circuit 206collectively operate as system control circuitry configured to determinewhen input switching criteria are met and controlling atrial event inputused by RV pacemaker 14 for detecting atrial events to provideatrial-synchronized ventricular pacing.

Power source 314 provides power to each of the other circuits andcomponents of RA pacemaker 12 as required. Power source 314 may includeone or more energy storage devices, such as one or more rechargeable ornon-rechargeable batteries. The connections between power source 314 andother pacemaker circuits and components are not shown in FIG. 4 for thesake of clarity.

Telemetry circuit 308 includes a transceiver 309 and antenna 311 fortransferring and receiving data via a radio frequency (RF) communicationlink. Telemetry circuit 308 may be capable of bi-directionalcommunication with external device 20 (FIG. 1) and RV pacemaker 14 asdescribed above. Telemetry circuit 308 may be configured to at leasttransmit wireless communication signals from RA pacemaker 12 to RVpacemaker 14 for signaling RV pacemaker 14 when an atrial pacing pulseor sensed P-wave has occurred. Coded wireless transmission signals maybe transmitted by telemetry circuit 308 to signal RV pacemaker when anintrinsic, sensed P-wave is detected and when an atrial pacing pulse hasbeen delivered.

FIG. 5 is an example of a motion sensor signal 250 that may be producedby motion sensor 212 of RV cardiac signal sensing circuit 204 and passedto atrial event detector circuit 240 of RV pacemaker 14. The motionsensor signal 250 shown represents one ventricular cycle. Verticaldashed lines 252 and 262 denote the timing of two consecutiveventricular events (an intrinsic ventricular depolarization or aventricular pace), marking the respective beginning and end of theventricular cycle 251. The motion signal includes an A1 event 254, an A2event 256, an A3 event 258 and an A4 event 260. The A1 event 254 is anacceleration signal that occurs during ventricular contraction and marksthe approximate onset of ventricular mechanical systole. The A2 event265 is an acceleration signal that occurs during ventricular relaxationand marks the approximate offset or end of ventricular mechanicalsystole. The A3 event 258 is an acceleration signal that occurs duringpassive ventricular filling and marks ventricular mechanical diastole.Since the A2 event occurs with the end of ventricular systole, it is anindicator of the onset of ventricular diastole. The A3 event occursduring ventricular diastole. As such, the A2 and A3 events may becollectively referred to as ventricular mechanical diastolic eventsbecause they are both indicators of the ventricular diastolic period.

The A4 event 260 is an acceleration signal that occurs during atrialcontraction and active ventricular filling and marks atrial mechanicalsystole. The A4 event 260 is an atrial systolic mechanical event thatmay be detected by atrial event detector circuit 242 as an atrial eventwhen the A4 events of motion sensor signal 250 are selected by RVpacemaker control circuit 206 as the atrial event input. The A4 event260 may be detected from motion sensor signal 250 by atrial eventdetector circuit 240 for controlling pace timing circuit 242 to triggerventricular pacing pulse delivery by starting the AV pacing interval inresponse to detecting the A4 event 260. Control circuit 206 may beconfigured to detect one or more of the A1, A2, and A3 events frommotion sensor signal 250, for at least some ventricular cardiac cycles,for use in positively detecting the A4 event 260 and setting atrialevent detection control parameters. The A1, A2 and/or A3 events may bedetected and characterized to avoid false detection of A4 events andpromote reliable A4 event detection for proper timing ofatrial-synchronized ventricular pacing pulses.

The A1, A2, A3 and/or A4 events may be evaluated by control circuit 206for determining the quality and reliability of motion sensor signal 250for atrial event detection. One or more signal quality metrics based onthe timing and/or morphology of the A1, A2, A3 and/or A4 events may beused in selecting an atrial event input by control circuit 206. In someexamples, unacceptable A4 event signal quality may be an input switchingcriterion causing RV pacemaker 14 to switch the atrial event input.Atrial event detector circuit 240 of RV pacemaker 14 may be configuredto detect the atrial event from the motion sensor signal 250 usingtechniques generally disclosed in U.S. Pat. Publication No. 2016/0023000A1 (Cho, et al.), U.S. patent application Ser. No. 15/140,585 filed Apr.28, 2016 (Ghosh, et al.), and U.S. patent application Ser. No.15/280,538 filed on Sep. 29, 2016 (Splett, et al.) and Ser. No.15/280,339 filed on Sep. 29, 2016 (Sheldon, et al.), all of which areincorporated herein by reference in their entirety.

FIG. 6 is a flow chart 350 of a method performed by intracardiac pacingsystem 10 for delivering atrial-synchronized ventricular pacingaccording to one example. At block 352, an initial atrial event input isselected by control module 206 of RV pacemaker 14. The initial atrialevent input may be far-field P-wave signals produced by RV pacemakersensing circuit 204 in the cardiac electrical signal received viaelectrodes 162V and 164V and passed to control circuit 206. In otherexamples, the initial atrial event input may be A4 event signals, alsoreferred to herein as “atrial systolic mechanical event signals” orsimply “atrial mechanical event signals,” in the motion signal producedby motion sensor 212, e.g., A4 event signals in motion sensor signal 250shown in FIG. 5.

The atrial event input initially selected at block 352 may be a defaultatrial event input that RV pacemaker 14 is programmed to initiallyselect. In other examples, the initial input may be selected based on ananalysis of signal quality metrics of one or more cardiac signals. Forinstance, the reliability of P-wave sensing from the cardiac electricalsignal received by RV sensing circuit 204 may be assessed by controlcircuit 206 by determining the P-wave amplitude, a ratio of P-wave toR-wave amplitude, a ratio of P-wave to T-wave amplitude, or other P-wavesignal strength metric or combination of metrics. The motion signalreceived from motion sensor 212 may be evaluated to determine if atrialsystolic mechanical event signals can be reliably detected based on theamplitude and/or timing of the atrial systolic event signal and/or theamplitude and/or timing of ventricular mechanical event signals includedin the motion signal.

The signal quality analysis for determining if atrial event detection isreliable may be performed on a first atrial event input, and ifacceptable the first atrial event input may be selected. If notacceptable, control circuit 206 may analyze the signal quality of asecond atrial event input for reliable atrial event detection. In otherexamples, two or more available atrial event inputs may be evaluated andthe input having the highest signal quality for atrial event detection,e.g., the signal with the greatest atrial event signal strength relativeto signal baseline and ventricular events, may be selected as theinitial atrial event input.

In other examples, the selection of an initial atrial event input may bebased at least in part on a patient condition, such as the paced orintrinsic atrial heart rate, patient physical activity level and/orpatient body posture. Reliability of P-wave sensing from the cardiacelectrical signal and/or atrial systolic mechanical event detection fromthe motion sensor signal may be posture dependent in some patients.Detection of physiological atrial events may be relatively morechallenging when the heart rate is elevated. Detection of A4 events fromthe motion sensor signal may be more challenging when patient physicalbody movement produces increased motion signals in the motion sensorsignal. Detection of P-waves and A4 events may be confounded byincreased heart rate when the timing between T-waves and P-waves orventricular mechanical diastolic events and atrial mechanical systolicevents becomes shorter or even fused.

As such, the patient's heart rate, physical activity and/or body posturemay be determined at block 352 and contribute to the initial atrialevent input selection according to which input is expected to provideacceptable atrial event detection. Patient physical activity and posturedetection may be determined from the motion sensor signal received fromsensor 212 using techniques generally described in theabove-incorporated U.S. Pat. No. 5,593,431 (Sheldon) and U.S. Pat. No.6,044,297 (Sheldon). During this process of selecting the initial atrialevent input, for example upon initial implantation of RA pacemaker 12and RV pacemaker 14, RA pacemaker 12 may temporarily broadcast atrialevent signals to enable RV pacemaker 14 to confirm P-wave and/or A4event detections and determine the atrial rate for contributing to theatrial event input selection. RV pacemaker control circuit 206 maydetermine the patient's physical activity and/or body posture from themotion sensor signal received from motion sensor 212. If the heart rateand/or patient physical activity exceed respective thresholds and/orpatient posture is identified as a poor condition for using aphysiological atrial event input, the initial atrial event input may beselected as a broadcast atrial event input.

Generally, powering RV pacemaker telemetry circuit 208 to receivebroadcast atrial event signals and powering RA pacemaker telemetrycircuit 308 to broadcast the atrial event signals on a beat-by-beatbasis, or nearly beat-by-beat basis, requires greater power consumptionthan detecting physiological atrial events from the cardiac electricalsignal or the motion sensor signal. In order to detect atrial pacingpulses as broadcast atrial events from the cardiac electrical signalreceived by RV pacemaker 14, the atrial pacing pulse energy delivered byRA pacemaker 12 may need to be increased and/or extra refractory pacingpulses may be delivered, which increases power consumption by RApacemaker 12.

As such, a physiological atrial event input selected at block 352 may bemore power efficient for the overall system 10 than an atrial eventinput that relies on broadcast atrial event signals generated by RApacemaker 12. The initial atrial event input signal may thereforepreferentially be a physiological atrial event input rather than abroadcast atrial event input that may require greater power consumptionin the overall pacing system 10. In some cases, however, e.g., duringinitial operation of the system 10, in the presence of electrical signalor motion sensor signal noise, high heart rate, high patient activity,or a detected patient body posture known to be associated with poorphysiological atrial event detection, a broadcast atrial event input maybe selected as the input at block 352 to promote reliable atrial eventdetection and proper timing of ventricular pacing pulses.

After selecting an atrial event input, atrial event detector circuit 240waits for an atrial event detection at block 354. If an atrial event isdetected, pace timing circuit 242 starts an AV pacing interval at block356. If an R-wave is sensed by sensing circuit 204 at block 358 beforeexpiration of the AV pacing interval (block 360), the scheduledventricular pacing pulse is withheld. Pace timing circuit 242 starts aventricular LR pacing interval at block 378 in response to the sensedR-wave.

If the AV pacing interval expires without sensing an R-wave, thescheduled ventricular pacing pulse is delivered by pulse generator 202at block 372. Control circuit 202 may determine if input switchingcriteria are met at block 374. Generally, if the atrial event isdetected and the ventricular pacing pulse delivered, it is expected thatthe selected atrial event input is reliable for continuing using theselected input for atrial event detection. As such, the determination atblock 374 may not be performed when the ventricular pacing pulse isdelivered at block 372 upon expiration of the AV pacing interval.

In some cases, however, a change in patient activity, patient posture,or atrial rate may be condition for causing the atrial event input to beswitched to another input considered to be more reliable under thepresent patient condition(s). As such, the atrial event rate, patientphysical activity level, patient body posture or other patient conditionmay be determined at block 374 for determining if input switchingcriteria are met. If input switching criteria are not met, the processreturns to start the ventricular LR pacing interval at block 378 inresponse to delivering the ventricular pacing pulse. If input switchingcriteria are met at block 374, a different atrial event input isselected at block 376.

If an atrial event is not detected at block 354 before an R-wave issensed at block 362, the AV interval is not started. If an R-wave issensed at block 362, before the ventricular LR pacing interval expires(block 370) and before an atrial event is detected (block 354), theventricular pacing pulse is inhibited block 364. At block 374, controlcircuit 206 determines if the input switching criteria are met. If anatrial event is not detected and an R-wave is not sensed (“no” branch ofblock 362) and the ventricular LR pacing interval expires (block 370), aventricular pacing pulse is delivered at block 372. A ventricular pacingpulse is delivered at the ventricular LR pacing interval to preventventricular asystole. After delivering the ventricular pacing pulse,control circuit 206 determines if the input switching criteria are metat block 374.

Input switching criteria may be satisfied at block 374 if atrial eventsare not detected for a predetermined number of consecutive ornon-consecutive ventricular cycles. In other words, if a predeterminednumber of ventricular pacing pulses are delivered at the ventricular LRpacing interval and/or R-waves are sensed prior to expiration of theventricular LR pacing interval, the atrial event detection may be deemedunreliable and the input switching criteria may be satisfied. Forexample, if 3 out of 5, 6 out of 8, 12 out 15 or other threshold numberof consecutive or non-consecutive ventricular cycles occur withoutdetecting the atrial event out of a predetermined number of consecutiveventricular cycles, the input switching criteria may be satisfied atblock 374. Atrial event input is switched at block 376 in response tothe input switching criteria being met. The atrial event input isswitched by enabling atrial event detector circuit 240 to detect ofatrial events from different input signals.

For example, atrial event detector circuit 240 may be receiving thecardiac electrical signal from sensing circuit 204 for sensing far-fieldP-waves and switch to receiving the motion signal from motion sensor 212for detecting atrial systolic mechanical event signals at block 376 orvice versa. Atrial event detector circuit 240 may switch from sensingP-waves or from sensing atrial systolic mechanical events to receivingatrial event communication signals from telemetry circuit 208. In otherinstances, atrial event detector circuit 240 switches from a currentatrial event input to detecting far-field atrial pacing pulses from thecardiac electrical signal passed to control circuit 206, e.g., a signalproduced by ADC 226 of sensing circuit 204.

Enabling a different atrial event input at block 376 may include actionstaken by RA pacemaker 12 in addition to switching the atrial event inputused by atrial event detector circuit 240 of RV pacemaker 14. As such,RA pacemaker 12 may be configured to monitor for atrial eventbroadcasting criteria being met at block 374 to enable RA pacemaker 12to recognize when RV pacemaker 14 is switching between atrial eventinputs. Recognition of the RV pacemaker 14 switching to a differentatrial event input by RA pacemaker 12 enables RA pacemaker 12 toinitiate broadcasting of atrial event signals by RA pacemaker telemetrycircuit 308, by increasing atrial pacing pulse energy delivered by RApacemaker pulse generator 302, and/or by delivering refractory atrialpacing pulses to facilitate detection of atrial events by atrial eventdetector circuit 240. It is recognized that in some cases, an increasedatrial pacing pulse energy is not required for RV pacemaker 14 to detectthe atrial pacing pulses.

FIG. 7 is a timing diagram 400 depicting cardiac events that are sensedby RA pacemaker 12 and RV pacemaker 14 during ventricular pacing.Timeline 402 depicts atrial events (A) 404 occurring at a stable atrialrate interval 403. The atrial events 404 may be intrinsic P-waves sensedby sensing circuit 304 of RA pacemaker 12, or the atrial events 404 maybe atrial pacing pulses delivered by RA pacemaker pulse generator 302 atan atrial LR pacing interval set at a permanent LR interval or a SIRbased on patient physical activity. RA pacemaker 12 senses ventricularevents (VS) 406 at a detected AV interval 408 that matches an expectedAV pacing interval 418 set by RV pacemaker pace timing circuit 242.

Timeline 412 depicts ventricular pacing pulses (VP) 416 and detectedatrial events (AS) 414 that are detected by atrial event detectorcircuit 240. Atrial events 414 may be detected as P-waves from thecardiac electrical signal produced by RV pacemaker sensing circuit 204or as A4 events detected from a motion sensor signal produced by motionsensor 212. Pace timing circuit 242 sets the AV pacing interval 418 inresponse to detecting the atrial events 414 and delivers the ventricularpacing pulse 416 upon expiration of AV pacing interval 418 (in theabsence of a sensed R-wave). The ventricular pacing pulse 416, or thesubsequently evoked R-wave, is detected by RA pacemaker 12 as VS event406 at the detected AV interval 408, which matches the expected AVpacing interval 418.

Atrial event detection is lost by RV pacemaker 14 when an expectedatrial event 415 is not detected by atrial event detector circuit 240.RV pacemaker 14 controls the pulse generator 202 to deliver aventricular pacing pulse 417 at the ventricular LR pacing interval 420following the preceding pacing pulse in the absence of a detected atrialevent. RA pacemaker 12 detects the ventricular event 407 at a detectedAV interval 409, which is greater than the expected AV pacing interval418.

In the example shown, three ventricular pacing pulses are delivered atthe ventricular LR pacing interval 420 in the absence of detected atrialevents. The corresponding detected AV intervals 409, 410 and 411 aredetermined by RA pacemaker 12 between respective atrial events andsensed ventricular events 407. These detected AV intervals 409, 410 and411 are progressively increasing. In the example shown the ventricularpacing pulses 417 delivered at the ventricular LR interval 420 occur ata slower rate than the atrial rate resulting in a progressivelyincreasing detected AV interval 409, 410 and 411. In some examples, RVpacemaker control circuit 206 may be configured to deliver theventricular pacing pulses using a rate smoothing algorithm in theabsence of detected atrial events. As such, the first ventricular pacingpulse 417 may be delivered later than the expected atrial event 415 butearlier than the programmed ventricular LR interval. Subsequentventricular pacing pulses may be delivered at progressively increasingLR intervals to gradually adjust the ventricular rate from theatrial-tracking rate to the programmed ventricular rate. In this case,the detected AV intervals 409, 410, and 411 will progressively increaseas the ventricular pacing interval is increased according to a ratesmoothing algorithm.

RA pacemaker control circuit 306 may be configured to produce broadcastatrial event signals in response to a threshold number of unexpecteddetected AV intervals. The threshold number may be one or moreunexpected detected AV intervals, which may be consecutive ornon-consecutive detected AV intervals. In the example shown, RApacemaker 12 enables an atrial event broadcast mode by broadcastingatrial event 405 after three unexpected detected AV event intervals 409,410, and 411, though another threshold number of unexpected detected AVintervals may be used.

The broadcast atrial event 405 is detected as AS event 422 by the atrialevent detector 240 of RV pacemaker 14, causing the AV pacing interval424 to be set for delivering the next ventricular pacing pulse 425. RApacemaker 12 senses the ventricular pacing pulse (or the subsequentlyevoked R-wave) as VS event 413 at the expected detected AV interval 421.Control circuit 306 of RA pacemaker 12 determines that atrial eventdetection by RV pacemaker 14 is restored based on the expected detectedAV interval 421. If the atrial broadcast event 405 is not followed by anexpected detected AV interval 421, RA pacemaker 12 may increase thestrength of the broadcast atrial event 405 or change the type ofbroadcast atrial event signal until restoration of atrial eventdetection by RV pacemaker 14 is recognized based on one or more expecteddetected AV intervals.

The AV pacing interval 424 and expected detected AV interval 421 may bethe same as the original AV pacing interval 418 and expected detected AVinterval 408, respectively, but may be different intervals. RV pacemakercontrol circuit 206 may set a different AV pacing interval 424 inresponse to detecting atrial event 422 from a broadcast atrial eventinput than the AV pacing interval 418 set in response to detectingatrial event 414 from a physiological atrial event input. The AV pacinginterval 424 may be adjusted to be different than AV pacing interval 418to account for a difference in timing of the detected atrial events 414and 422 relative to the actual, corresponding atrial events. Forexample, an inherent system delay may be present in detecting broadcastatrial event signal 405 compared to detecting a physiological atrialevent input signal. After switching to the broadcast atrial event input,RV pacemaker 14 may adjust the AV pacing interval 418 according to whichbroadcast signal is being used as the atrial event input. For instance,when the atrial event is detected by receiving a broadcast wirelesscommunication signal 405 by telemetry circuit 208, pace timing circuit242 may set the AV pacing interval 424 to be 10 to 50 ms shorter thanthe AV pacing interval 418 used when the atrial event is detected as aP-wave from the cardiac electrical signal. If the atrial event input isinitially the A4 event signals of the motion sensor signal, the AVpacing interval 424 may be set longer than the AV pacing interval 418. Arelatively short AV pacing interval 418 may be set following an A4 eventdetection, e.g., 50 ms, which occurs later than the atrial electricalevents. If broadcast atrial event 405 is a delivered atrial pacingpulse, the AV pacing interval 424 may be longer than AV pacing interval418, e.g., increased from 50 ms to 200 ms.

Different AV pacing intervals may be used when the atrial events arebeing detected from different physiological atrial event inputs and whenthe atrial events are being detected from different broadcast atrialevent inputs. A physiological time delay is expected between an atrialpacing pulse and a sensed P-wave and between the sensed P-wave and theatrial mechanical contraction. As such, the AV pacing interval used bypace timing circuit 242 may be selected based on the atrial event inputbeing used by atrial event detector circuit 240 to account forphysiological time delays and/or intracardiac pacemaker system timedelays that may exist in detecting broadcast atrial event signals.

The atrial broadcast event 405 may be a wireless communication signaltransmitted by RA pacemaker telemetry circuit 308 upon sensing an atrialP-wave by sensing circuit 304 or upon delivering an atrial pacing pulseby pulse generator 302. In this case, the detected atrial event 422 iswireless atrial event communication signal received by RV pacemakertelemetry circuit 208 that is passed to atrial event detector circuit240 (or directly to pace timing circuit 242). RV pacemaker 14 may beconfigured to switch the atrial event input to broadcast communicationsignals received from RA pacemaker telemetry circuit 308 via RVpacemaker telemetry circuit 208 in response to a threshold number ofventricular cycles paced at the ventricular LR pacing interval withoutatrial event detections. The threshold number of paced ventricularcycles at the LR pacing interval may or may not be required to beconsecutive.

In the example of FIG. 7, RV pacemaker telemetry circuit 208 is enabledto receive communication signals in response to three consecutiveventricular pacing pulses delivered at the ventricular LR pacinginterval (or rate smoothing intervals). The threshold number of lostatrial event detections required for input switching criteria to be metin RV pacemaker 14 may or may not be the same as the threshold number ofunexpected detected AV intervals required for RA pacemaker 12 to enablebroadcast atrial event signals. Control circuit 206 may enable telemetrycircuit 208 to be powered up to a listening mode for receiving atrialevent broadcast signals from the RA telemetry circuit 308.

In another example, the atrial broadcast event 405 may be an atrialpacing pulse that is delivered at an increased pacing pulse amplitudeand/or pulse width. Atrial events 404 may be atrial pacing pulses thatare delivered at a safety margin greater than an atrial pacing capturethreshold. In response to a threshold number of unexpected detected AVintervals, e.g., intervals 409, 410 and 411, RA pacemaker controlcircuit 306 may increase the RA pacing pulse energy by controlling pulsegenerator 302 to deliver pacing pulses with a pulse amplitude and/orpulse width that is greater than the pulse amplitude and/or widthnormally used to deliver atrial pacing pulses when RA pacemaker 12 isnot broadcasting atrial event signals.

In some examples, the atrial pacing pulses may not be required to beincreased in order to be detected by RV pacemaker 14. In this case, thebroadcast atrial event signal may be a double pacing pulse, one tocapture the atria and a second refractory pacing pulse to signal RVpacemaker 14 that the atrial event is a paced event. A single refractorypacing pulse may be delivered by RA pacemaker 12 as a broadcast atrialevent when an intrinsic P-wave is sensed to signal RV pacemaker 14 thatthe atrial event is a sensed event.

RV pacemaker control circuit 206 may switch the atrial event input byenabling atrial event detector circuit 240 to detect far-field atrialpacing pulses from the cardiac electrical signal produced by sensingcircuit 204. Example techniques for detecting pacing pulses from acardiac electrical signal are generally disclosed in pending U.S.Publication No. 2016/0250478 (Greenhut, et al.) and U.S. patentapplication Ser. No. 14/826,396 filed Aug. 14, 2015 (Gunderson, et al.),both incorporated herein by reference in its entirety.

RA pacemaker control circuit 306 may determine whether to broadcastatrial event signals as wireless telemetry communication signals or bydelivering atrial pacing pulses (at a higher pacing pulse energy and/orrefractory pacing pulses) or a combination of both communication signalsand pacing pulses during the atrial event broadcast mode. If the atrialrhythm is a paced rhythm at the time that atrial event detection by RVpacemaker 14 is determined to be lost by RA pacemaker 12, RA pacemaker12 may broadcast atrial events by delivering the atrial pacing pulses atthe same or an increased pulse energy. If the atrial rhythm is a sensedrhythm at the time that atrial event detection by RV pacemaker 14 islost, RA pacemaker 12 may broadcast the time of the sensed atrial eventsby transmitting a wireless communication signal via telemetry circuit308 or by delivering refractory pacing pulses. In some examples, RApacemaker 12 broadcasts atrial event signals using a combination ofpacing pulses and wireless communication signals transmitted bytelemetry circuit 308. Atrial pacing pulses, when delivered, may bedelivered at the normal or increased pacing pulse energy to signal pacedatrial events. If a P-wave is sensed, the atrial event signal may bebroadcast by the telemetry circuit 308. In this case, RV pacemaker 14enables atrial event detector circuit 240 to monitor for both far-fieldatrial pacing pulses in the cardiac electrical signal received fromsensing circuit 204 and for wireless communication signals received byRV pacemaker telemetry circuit 208.

In some examples, RV pacemaker control circuit 206 may enable atrialevent detector circuit 240 to switch from one physiological atrial eventinput to another physiological atrial event input, if available, beforerelying on a broadcast atrial event input, which may require greaterpower consumption by one or both of pacemakers 12 and 14 than use of aphysiological atrial event input. For instance, RV pacemaker 14 mayinitially select P-wave sensing from the cardiac electrical signal asthe atrial event input. If P-wave sensing is lost for a threshold numberof ventricular cycles, RV pacemaker 14 may switch to A4 event detectionfrom the motion sensor signal as the atrial event input. Both of theseatrial event inputs and atrial event detection methods do not require RApacemaker 12 to alter its operation. Atrial event signals that areselected as atrial event input without requiring RA pacemaker 12 toalter its operation are referred to herein as “physiological” atrialevent input signals. The physiological atrial event signals arise from aphysiological source as opposed from originating from the RA pacemaker12 as a device-generated atrial event signal. If atrial event detectionis not restored after switching from one physiological atrial eventinput to another physiological atrial event input, e.g., from P-wavesignals to atrial mechanical event signals or vice versa, RV pacemaker14 may enable atrial event detection using broadcast atrial event inputoriginating from RA pacemaker 12.

RV pacemaker 14 may switch from a first physiological atrial event inputto a second physiological atrial event input in response to a firstthreshold number of ventricular cycles without atrial event detections.If a second threshold number of ventricular cycles occur without atrialevent detections after switching the second physiological atrial eventinput, RV pacemaker 14 may switch atrial event input used by atrialevent detector circuit 240 from the second physiological atrial eventinput to a broadcast atrial event input. RA pacemaker 12 may beconfigured to enable atrial event broadcasting in response to the secondthreshold number of unexpected detected AV intervals. In other examples,RA pacemaker 12 may be enabled to broadcast atrial event signals when athird threshold number of unexpected detected AV intervals is detectedthat is intermediate the first and second thresholds used by RVpacemaker 14. In this way, by the time RV pacemaker 14 switches thebroadcast atrial event input, RA pacemaker 12 will already bebroadcasting atrial event signals for immediate detection by RVpacemaker 14.

FIG. 8 is a timing diagram of atrial events and ventricular eventsproduced and detected by intracardiac pacemaker system 10. Timeline 452depicts events produced and detected by RA pacemaker 12. Timeline 454depicts events produced and detected by RV pacemaker 14. RA pacemaker 12is operating in an atrial event broadcast mode, producing broadcastatrial event signals 405. As described above in conjunction with FIG. 7,atrial event detector circuit detects the broadcast atrial events 405and produces an atrial event sense signal (AS) 422. Pace timing circuit242 starts an AV pacing interval 424 and controls pulse generator 202 todeliver a pacing pulse 425 at the expiration of AV pacing interval 424.RA pacemaker 12 detects the ventricular pacing pulse (or the evokedR-wave) and as indicated by VS event 413 at the expected detected AVinterval 421.

RA pacemaker control circuit 306 may be configured to periodicallywithhold an atrial event broadcast signal to determine if atrial eventdetection by RV pacemaker 14 using a physiological atrial event inputhas returned. The atrial event 454 may be an atrial pacing pulse that isdelivered at normal pacing pulse energy, e.g., at a safety margingreater than a determined atrial pacing capture threshold. In otherinstances, atrial event 454 may be a sensed P-wave. A broadcast atrialevent signal indicating the time of atrial event 454 is withheld by RApacemaker 12.

In this example, atrial event detection by RV pacemaker 14 may beswitched to a physiological atrial event input after detecting the Nthatrial event 427 using a broadcast atrial event input. The expectedatrial event detection 465 does not occur; atrial event detection usingthe physiological atrial event input has not returned. As a result, theventricular LR pacing interval 420 expires without an atrial eventdetection, and a ventricular pacing pulse 464 is delivered. Theventricular pacing pulse 464 (or an evoked far-field R-wave) is detectedby RA pacemaker 12 as VS event 458 at an unexpected detected AV interval456. Since the detected AV interval does not match the expected AVpacing interval 424, RA pacemaker 12 may return to an atrial eventbroadcast mode on the next atrial cycle, as indicated by broadcastatrial event 472. In other examples, the broadcast atrial event may bewithheld for multiple atrial cycles to enable RV pacemaker 14 to attemptatrial event detection on more than one ventricular cycle, perhaps usingmore than one physiological atrial event input or by adjustingphysiological atrial event detection parameters.

After a predetermined number of atrial event broadcast signals, e.g.,two or more broadcast signals, RA pacemaker 12 withholds the atrialevent broadcast signal again and RV pacemaker 14 switches to aphysiological atrial event input. This time, RV pacemaker 14 detects theatrial event 455 and produces an atrial sense event signal 467. RVpacemaker 14 may be programmed to switch from a broadcast atrial eventinput to a physiological atrial event input after detecting apredetermined number of broadcast atrial events so that the RA pacemaker12 and the RV pacemaker 14 are coordinated to respectively terminatebroadcasting atrial event signals and switch to a physiological atrialevent input on the same cardiac cycle(s).

RA pacemaker 12 detects a ventricular event 478 at a detected AVinterval 476 that meets expected detected AV interval criteria,confirming that detection of atrial events by RV pacemaker 14 has beenrestored. RA pacemaker 12 terminates the atrial event broadcast mode andcontinues delivering atrial pacing pulses at the normal pacing pulseenergy and sensing P-waves without broadcasting atrial event signals.

In the example shown, RV pacemaker 14 is configured to deliver the firstventricular pacing pulse 482 at a shortened AV pacing interval 480 afterthe detecting atrial event 455 using a physiological atrial event input.RA pacemaker 12 is configured to compare a detected AV pacing interval476 after withholding a broadcast atrial event signal to a temporary,shortened AV pacing interval 480. By delivering the first pacing pulse482 at a shortened AV pacing interval 480, RA pacemaker 12 confirms thatatrial event detection by RV pacemaker 14 has been restored based on theshortened, detected AV interval 476. In response to detecting theshortened AV interval 476, RA pacemaker 12 terminates the atrial eventbroadcast mode. On the next cardiac cycle, RV pacemaker 14 continuesdetecting atrial events from the physiological atrial event input and isconfigured to resume using the target AV pacing interval 418corresponding to the selected atrial event input for controllingventricular pacing pulse delivery.

FIG. 9 is a flow chart 500 of a method performed by RV pacemaker 14 forcontrolling atrial event input during atrial-synchronized ventricularpacing according to one example. FIG. 10 is a flow chart 600 of a methodperformed by RA pacemaker 12 operating in cooperation with the RVpacemaker 14 performing the methods of FIG. 9. RA pacemaker 12 isconfigured to determine appropriate times for broadcasting atrial eventsignals to support atrial event detection by RV pacemaker 14 when aphysiological atrial event input is not reliable for atrial eventdetection.

At block 502, RV pacemaker control circuit 206 selects an atrial eventinput used by atrial event detector circuit 240 for detecting atrialevents. Control circuit 206 sets the AV pacing interval at block 504based on the atrial event input selected at block 502. The initialatrial event input may be selected to be A4 events in the motion signalfrom motion sensor 212. In this case, the AV pacing interval may be setrelatively short, e.g., less than 100 ms or 50 ms or less. If theinitial atrial event input is selected to be P-wave signals in thecardiac electrical signal, the AV pacing interval may set relativelylonger, e.g., greater than 100 ms, such as around 200 to 250 ms. It isrecognized that in either case, the AV pacing interval may be anadjustable interval that changes with heart rate determined fromdetected atrial events.

The initial atrial event input selected at block 502 is typically aphysiological atrial event input, such as P-waves or A4 events, whichare not arising from a device-generated source. However, in somecircumstances, the atrial event input initially selected at block 502may be broadcast atrial event signals in which case control circuit 206may set the AV pacing interval to an interval that accounts fordifferences in relative timing of broadcast atrial events andphysiological atrial event signals and/or system delays that may beinherently present between the time of an actual atrial event and thetime the RV pacemaker 14 detects the broadcast signal.

At block 506, RV pacemaker 14 operates to deliver atrial-synchronizedventricular pacing using the selected atrial event input andcorresponding AV pacing interval. RV pacemaker control circuit 206controls the pulse generator 202 to deliver ventricular pacing pulsesupon expiration of the selected AV pacing interval following atrialevents detected from the selected atrial event input.

At block 508, control circuit 206 determines if atrial event detectionis lost. Atrial event detection may be determined to be lost when athreshold number of ventricular pacing pulses are delivered at theventricular LR pacing interval, without atrial event detection duringthe ventricular cycle. For example, atrial event detection may bedetermined to be lost when at least three consecutive or non-consecutiveventricular pacing pulses delivered at the ventricular LR pacinginterval. The ventricular LR pacing interval may be an adjusted ortemporary LR pacing interval to provide rate smoothing to avoid a suddenrate change in some examples.

If atrial event detection is determined to be lost at block 508, controlcircuit 206 may select a different atrial event input at block 510 fordetecting the atrial events, which may be a different physiologicalatrial event input when available. For example, if the atrial eventinput selected at block 502 was atrial systolic mechanical events, theatrial event input may be switched to P-wave signals, or vice versa.

Control circuit 206 determines if atrial event detection is restored atblock 512 using a different physiological atrial event input. Atrialevent detection may be determined to be restored at block 512 inresponse to a threshold number of atrial event detections, which may berequired to be consecutive but may be non-consecutive in some examples,e.g., 6 atrial event detections out of 8 consecutive ventricular cycles.If atrial event detection is restored by using a different physiologicalatrial event input, the AV pacing interval is adjusted at block 514according to the atrial event input now being used to maintain thedesired synchrony between atrial and ventricular systolic events. Theprocess may return to block 508 such that if atrial event detection islost again, the atrial event input may be switched back to the firstphysiological atrial event input or another physiological atrial eventinput if available, at block 510.

If atrial event detection is not restored from a physiological atrialevent input, control circuit 206 switches to a broadcast atrial eventinput. Control circuit 206 may select the broadcast atrial event inputby powering up telemetry circuit 208 for receiving broadcast atrialevent communication signals. Additionally or alternatively, controlcircuit 206 may select the broadcast atrial event input by enablingatrial event detector circuit 240 to detect far-field atrial pacingpulses from the cardiac electrical signal produced by sensing circuit204 and passed to control circuit 206

At block 518, control circuit 206 may control pulse generator 202 todeliver at least one ventricular pacing pulse at a temporary AV pacinginterval in response to detecting the broadcast atrial event. Thetemporary AV pacing interval may be shorter than the AV pacing intervalthat is normally selected for use during the selected atrial event inputand is used to signal to RA pacemaker 12 that an atrial event has beendetected. A temporary AV pacing interval may be used for one or morecardiac cycles each time the atrial event input is switched to enable RApacemaker 12 to confirm atrial event detection by RV pacemaker 14 isoccurring. The AV pacing interval is then adjusted at block 520 to theAV pacing interval that promotes optimal atrioventricular synchrony whena broadcast atrial event input is being used. In other examples, the AVpacing interval may be adjusted at block 520 based on the selectedbroadcast atrial event input without using a temporary AV pacinginterval. RV pacemaker 14 operates to deliver atrial-synchronizedventricular pacing using the selected broadcast atrial event input.

In some cases, the RA pacemaker 12 may broadcast a high energy atrialpacing pulse and/or a refractory pacing pulse on paced atrial cycles andbroadcast a wireless communication signal via telemetry circuit 308 whenan intrinsic atrial P-wave is sensed. In other cases, RA pacemaker 12may transmit two different, coded wireless communication signals bytelemetry circuit 308, one indicating when an atrial paced event occursand a different one when an atrial sensed event occurs. In still otherexamples, RA pacemaker 12 may deliver a double pacing pulse (one tocapture the RA and one delivered within the absolute refractory periodof the RA) to indicate a paced atrial event and deliver a signal pacingpulse during the absolute atrial refractory period following a sensedP-wave to indicate a sensed atrial event. RV pacemaker control circuit206 may set the AV pacing interval at block 520 to either a paced AVpacing interval or a sensed AV pacing interval according to the detectedbroadcast atrial event.

Control circuit 206 may determine when a predetermined number ofbroadcast atrial events have been detected at block 522, and switch backto a physiological atrial event input at block 524. Control circuit 206determines if atrial event detection using the physiological atrialevent input is restored at block 512. If more than one physiologicalatrial event input is available, each available physiological input maybe tested at block 512 until a physiological input is identified thatresults in reliable atrial event detection. If atrial event detectionfrom a physiological atrial event input is not restored, atrial eventdetector circuit 240 may continue detecting atrial events from broadcastatrial event input (block 516). If atrial event detection from thephysiological atrial event input is restored, RV pacemaker 14 adjuststhe AV pacing interval according to the selected atrial event input andreturns to block 506 for delivering atrial-synchronized ventricularpacing using the selected atrial event input.

The operations of RA pacemaker 12 performed during the operations of RVpacemaker 14 described in conjunction with FIG. 9 are shown by the flowchart of FIG. 10. At block 602, RA pacemaker control circuit 306 detectsa far-field ventricular event, e.g., a far-field R-wave or a ventricularpacing pulse, from the cardiac electrical signal received by sensingcircuit 304. Control circuit 306 determines the detected AV interval atblock 604 between the far-field electrical event and a most recentatrial paced or sensed event.

The detected AV interval is compared to the expected AV pacing intervalat block 606. RA pacemaker 12 may be programmed to store the AV pacingintervals used by RV pacemaker 14 for each corresponding atrial eventinput. RA pacemaker 12 may retrieve the AV pacing intervals programmedfor use for each of the available physiological atrial event inputs andcompare the detected AV interval to each of the possible AV pacingintervals that RV pacemaker 14 is expected to be using if RV pacemaker14 detected the most recent atrial event from a physiological atrialevent input. The detected AV interval may be compared to one or morepossible AV pacing interval ranges corresponding to each physiologicalatrial event input available in RV pacemaker 14.

If the detected AV interval matches a stored AV pacing interval expectedto be used by RV pacemaker during physiological atrial event input (atblock 606), e.g., within a predefined matching range, RA pacemaker 12continues to detect far-field ventricular events by returning to block602 and monitoring the detected AV intervals. The detected AV intervalmay be determined to match an expected AV pacing interval when thedetected AV interval is within a predetermined range, e.g., within 10ms, within 20 ms or within 50 ms or less, than an expected AV pacinginterval. If the detected AV interval does not match an expected AVpacing interval, the RA pacemaker 12 begins to broadcast atrial eventsignals at block 608.

The atrial event signals may be broadcast as described above. Forexample, broadcast atrial event signals may include coded wirelesscommunication signals indicating either a paced or sensed atrial event,atrial pacing pulses and/or atrial refractory pacing pulses, both ofwhich may be delivered using increased pacing pulse energy greater thanthe pacing pulse energy used to pace the atria when RA pacemaker 12 isnot broadcasting atrial event signals.

Upon broadcasting the first atrial event signal, RA pacemaker controlcircuit 306 determines the AV interval as the time from the broadcastatrial event signal (or corresponding atrial event) to the next detectedfar-field ventricular event at block 610 and compares the detected AVinterval to an expected AV interval. The expected AV interval may be aprogrammed AV pacing interval that RV pacemaker 14 is expected to usewhen the broadcast atrial event signal is detected. The expected AVinterval may be a temporary, shortened AV pacing interval as describedin conjunction with block 518 of FIG. 9.

If the detected AV interval matches an expected AV interval, asdetermined at block 610, e.g., within a matching range of ±10 to 20 ms,the process advances to block 614. If not, RA pacemaker 12 may adjustthe broadcast atrial event signal at block 612 to increase thelikelihood of being detected by RV pacemaker 14. RA pacemaker 12 mayadjust the broadcast atrial event signal by changing the type of signal(e.g., from a transmitted communication signal to an atrial pacing pulseor vice versa) or increasing the amplitude or strength of the broadcastatrial signal.

Once detection of the broadcast atrial event signal by RV pacemaker 14is confirmed, based on the detected AV interval matching the expected AVinterval at block 610, RA pacemaker 12 broadcasts a predetermined numberof atrial event signals. After the predetermined number of atrial eventsignals are broadcast, RA pacemaker 12 may withhold one or morebroadcast atrial event signals at block 614. RA pacemaker controlcircuit 306 determines the detected AV interval on the one or morecardiac cycles that the broadcast atrial event signal is withheld andcompares the detected AV interval to the expected AV pacing interval atblock 616. If the detected AV interval matches the expected AV pacinginterval, RA pacemaker 12 may terminate broadcasting atrial eventsignals at block 618 and return to block 602 for monitoring detected AVintervals for detecting a loss of atrial event detection by RV pacemaker14. If the detected AV interval does not match the expected AV interval,RA pacemaker resumes broadcasting atrial event signals at block 608.

In other examples, rather than withholding the broadcast atrial eventsignal(s) at block 614, RA pacemaker 12 may monitor the detected AVintervals at block 616 for detecting a temporary, shortened AV intervalwhile still broadcasting atrial event signals. RV pacemaker 14 may beconfigured to continue detecting the broadcast atrial event signals, butperiodically check if atrial event detection from a physiological atrialevent input is restored (blocks 514 and 512 of FIG. 9). Atrial eventdetector circuit 240 may be configured to detect atrial events from botha physiological atrial event input and the broadcast atrial event inputfor one or more cycles to maintain proper AV synchrony while checking ifatrial event detection from a physiological atrial event input isrestored. If restored, RV pacemaker 14 may signal RA pacemaker 12 thatAV event signal broadcasting is no longer required by delivering one ormore ventricular pacing pulses at a “coded” temporary, shortened AVpacing interval. The RA pacemaker detects the “coded” shortened AVpacing interval and terminates atrial event broadcasting at block 618.RV pacemaker 14 switches the atrial event input to the physiologicalinput and adjusts the AV pacing interval as needed at block 514 (FIG.9).

Thus, various methods for controlling atrial event input and detectionin an intracardiac pacemaker system configured to deliver ventricularpacing in an atrial-synchronized pacing mode have been describedaccording to illustrative embodiments. In other examples, variousmethods described herein may include steps performed in a differentorder or combination than the illustrative examples shown and describedherein. Furthermore, other circuitry may be conceived by one of ordinaryskill in the art for implementing the techniques disclosed herein; theparticular examples described herein are illustrative in nature and notintended to be limiting. It is appreciated that various modifications tothe referenced examples may be made without departing from the scope ofthe disclosure and the following claims.

The invention claimed is:
 1. An intracardiac pacemaker system,comprising: a ventricular intracardiac pacemaker; and a secondimplantable device configured to broadcast atrial event signals to theventricular intracardiac pacemaker, the ventricular intracardiacpacemaker comprising: a first pulse generator configured to generate anddeliver pacing pulses to a ventricle of a patient's heart via a firstplurality of electrodes coupled to the ventricular intracardiacpacemaker; a first sensing circuit configured to produce physiologicalatrial event signals; a receiving circuit configured to receive thebroadcast atrial event signals from the second implantable medicaldevice; a first control circuit coupled to the pulse generator, thefirst sensing circuit, and the receiving circuit, the first controlcircuit configured to: select a first atrial event input as thephysiological atrial event signals that do not require the secondimplantable device to broadcast or generate atrial event signals; detectfirst atrial events from the selected first atrial event input;determine if input switching criteria are met; switch from the firstatrial event input to a second atrial event input in response to theinput switching criteria being met, the second atrial event input beingthe broadcast atrial event signals; detect second atrial events from thesecond atrial event input; and set an atrioventricular (AV) pacinginterval in response to detecting each of the first atrial events andthe second atrial events for controlling the first pulse generator todeliver the ventricular pacing pulses.
 2. The system of claim 1, whereinthe first sensing circuit comprises a motion sensor configured toproduce first physiological atrial event signals as atrial mechanicalevent signals.
 3. The system of claim 2, wherein the first sensingcircuit is configured to produce second physiological atrial eventsignals different than the first physiological atrial event signals,wherein the first control circuit is configured to select the firstatrial event input by selecting one of the atrial mechanical eventsignals and the second physiological atrial event signals.
 4. The systemof claim 1, wherein the first sensing circuit comprises: a cardiacelectrical signal sensing circuit configured to receive a cardiacelectrical signal via the first plurality of electrodes, the cardiacelectrical signal comprising near-field R-waves and far-field P-wavesignals; and a motion sensor configured to produce a motion signalcomprising atrial mechanical event signals; wherein the first controlcircuit is configured to select the first atrial event input as one ofthe far-field P-wave signals and the atrial mechanical event signals. 5.The system of claim 1, wherein the second implantable medical device isan atrial intracardiac pacemaker comprising: a second pulse generatorconfigured to generate and deliver pacing pulses to an atrium of thepatient's heart via a second plurality of electrodes coupled to theatrial intracardiac pacemaker; a second sensing circuit configured toreceive a cardiac electrical signal comprising near field P-waves andfar-field ventricular event signals; and a second control circuitcoupled to the second pulse generator and the second sensing circuit andconfigured to: detect far-field ventricular event signals from thesecond cardiac electrical signal; detect a loss of detecting the firstatrial events by the ventricular intracardiac pacemaker based on thedetected far-field ventricular event signals; and produce the broadcastatrial event signals in response to detecting the loss.
 6. The system ofclaim 5, wherein the second control circuit is configured to: controlthe second pulse generator to deliver first atrial pacing pulses at afirst pacing pulse energy; and produce the broadcast atrial eventsignals by controlling the second pulse generator to deliver secondatrial pacing pulses at a second pacing pulse energy greater than thefirst pacing pulse energy, wherein the receiving circuit receives thebroadcast atrial event signals as far-field atrial pacing pulse signals,and the first control circuit is configured to switch from the firstatrial event input to the second atrial event input by detecting thefar-field atrial pacing pulse signals as the second atrial events. 7.The system of claim 5, wherein: the receiving circuit comprises a firsttelemetry circuit included in the ventricular intracardiac pacemaker;the atrial intracardiac pacemaker comprises a second telemetry circuit;the second control circuit is configured to produce the broadcast atrialevent signals by controlling the second telemetry circuit to broadcastthe atrial event signals; wherein the first control circuit isconfigured to switch from the first atrial event input to the secondatrial event input by enabling the first telemetry circuit to receivethe broadcast atrial event signals and detect the received broadcastatrial event signals as the second atrial events.
 8. The system of claim5, wherein the second control circuit is further configured to: withholda broadcast atrial event signal coinciding with a next atrial eventafter a predetermined number of broadcast atrial event signals; sense afar-field ventricular event following the next atrial event; determine atime interval from the next atrial event to the sensed far-fieldventricular event; compare the time interval to an expectedatrial-to-ventricular interval; terminate broadcasting of the atrialevent signals in response to the time interval matching the expectedatrial-to-ventricular interval; and return to broadcasting the atrialevent signals in response to the time interval not matching the expectedatrial-to-ventricular interval.
 9. The system of claim 8, wherein thefirst control circuit is configured to: switch to a third atrial eventinput after detecting the predetermined number of broadcast atrial eventsignals, the third atrial event input comprising physiological atrialevent signals; detect the next atrial event from the third atrial eventinput; set the AV pacing interval to a temporary interval in response todetecting the next atrial event; control the first pulse generator todeliver a ventricular pacing pulse upon expiration of the temporaryinterval; wherein the expected atrial-to-ventricular interval is thetemporary interval.
 10. The system of claim 1, wherein the first controlcircuit is further configured to set the AV pacing interval to a firstinterval in response to detecting each of the first atrial events andset the AV pacing interval to a second interval different than the firstinterval in response to detecting each of the second atrial events. 11.The system of claim 1, wherein the first control circuit is configuredto determine if the input switching criteria are met by at least one of:detecting a threshold number of ventricular cycles without first atrialevent detections; determining a heart rate of the patient; determining apatient posture from a motion signal produced by the first sensingcircuit; and determining a patient physical activity level from a motionsignal produced by the first sensing circuit.
 12. The system of claim 1,wherein: the first sensing circuit comprises a first sensor configuredto produce first physiological event signals and a second sensorconfigured to produce second physiological atrial event signals; thefirst control circuit is further configured to: determine if less than afirst threshold number of first atrial events are detected from thefirst physiological atrial event signals during a first predeterminednumber of ventricular cycles; enable detection of the first atrialevents from the second physiological atrial event signals in response toless than the first threshold number of first atrial events beingdetected from the first physiological atrial event signals; anddetermine that the input switching criteria are met in response to lessthan a second threshold number of first atrial events being detectedfrom the second physiological atrial event signals during a secondpredetermined number of ventricular cycles.
 13. The system of claim 5,wherein: the second control circuit is configured to produce a firstbroadcast atrial event signal indicating an atrial pacing pulse eventand a second broadcast atrial event signal indicating an intrinsicatrial event; the first control circuit is further configured to set theAV pacing interval to a first interval in response to detecting thefirst broadcast atrial event signal and set the AV pacing interval to asecond interval different than the first interval in response todetecting the second broadcast atrial event signal.
 14. A ventricularintracardiac pacemaker, comprising: a pulse generator configured togenerate and deliver pacing pulses to a ventricle of a patient's heartvia a first plurality of electrodes coupled to the ventricularintracardiac pacemaker; a sensing circuit comprising a sensor configuredto produce physiological atrial event signals; a receiving circuitconfigured to receive atrial event signals broadcast from anothermedical device; a control circuit coupled to the pulse generator, thesensing circuit, and the receiving circuit, the control circuitconfigured to: select a first atrial event input as the physiologicalatrial event signals that do not require another medical device tobroadcast or generate atrial event signals; detect first atrial eventsfrom the selected first atrial event input; determine if input switchingcriteria are met; switch from the first atrial event input to a secondatrial event input in response to the input switching criteria beingmet, the second atrial event input being the broadcast atrial eventsignals; detect second atrial events from the second atrial event input;and set an atrioventricular (AV) pacing interval in response todetecting each of the first atrial events and the second atrial eventsfor controlling the first pulse generator to deliver the ventricularpacing pulses.